How serious is an emission control system code?

Most emission codes are moderate in severity. While they may not immediately affect drivability, they increase harmful exhaust emissions and should be repaired to maintain vehicle compliance and performance.

Can I fix emission codes myself?

Some emission issues like loose hoses or simple valve replacements can be DIY repairs. However, diagnosing the root cause often requires specialized equipment, so a professional diagnosis is recommended.

P0491

Secondary Air Injection System Insufficient Flow (Bank 1)

Moderateemission

Overview

The P0491 code indicates the PCM has identified a secondary air injection system insufficient flow (bank 1) issue. This code is associated with the vehicle's emission control systems designed to reduce harmful exhaust pollutants. A malfunction in these systems can increase vehicle emissions beyond acceptable limits and will likely cause a failed emissions inspection.

Symptoms

Unusual exhaust smell
Failed emissions test
Reduced engine performance
Hissing or sucking sound from engine bay

Common Causes

Faulty air injection switching valve
Damaged or disconnected air injection hoses
Failed relay in the air injection system
Failed check valve in air injection system

Diagnosis Steps

1Connect an OBD-II scanner and read all stored trouble codes and freeze frame data
2Inspect the secondary air injection pump for proper operation
3Check all hoses and pipes in the air injection system for damage or leaks
4Test the switching valve solenoid and relay with a multimeter
5Verify airflow through the system using a flow meter if available

Estimated Repair Cost

$200 — $700

Cost varies by vehicle, location, and repair shop. Includes parts and labor.

Commonly Affected Vehicles

Honda CR-VSubaru OutbackFord FocusVolkswagen Passat

Related Codes

P0420 P0412 P0411

More Emission Codes

P0410Secondary Air Injection System Malfunction

Moderate

P0411Secondary Air Injection System Incorrect Flow Detected

Moderate

P0412Secondary Air Injection System Switching Valve "A" Circuit

Moderate

P0413Secondary Air Injection System Switching Valve "A" Circuit Open

Moderate

P0414Secondary Air Injection System Switching Valve "A" Circuit Shorted

Moderate

P0415Secondary Air Injection System Switching Valve "B" Circuit

Moderate

Frequently Asked Questions

Yes, any illuminated Check Engine Light will result in a failed emissions inspection. The emission system issue must be repaired and the code cleared before the vehicle can pass.